I forgot my ISO was a combined Home/Pro version and that's why it is large.

The 1511 install from booting the UFD to the Win10 desktop took 9 minutes. Fast. I clicked "I don't have a Product Key". I also clicked "Skip this step" when asked for a Microsoft account. Skipping lets you use a Local account.

when does the system finally ask for the Product Key? When first connecting to the internet and MS servers?

There are no product keys for Win10 as far as I know. I just checked my recent 1511 install. It is Activated because I've previously activated a Win10 on this computer. A Win8 to Win10 upgrade. The activation for Win7/8 to Win10 just happens. You don't have to do anything.

This is a multi-boot computer. I have 5 Win10 installed. All are activated.

My HDD is split in three partitions - partition 1 (primary) with WinXP, partition 2 (primary) with Win7 and partition 3 (logical on extended) with data. All partitions were created and formated during setup of WinXP to make Ghost 2003 work (partition alignment). There's no "200 MB system partition" but all that has ended up on partition 1, I guess.

Now, upgrading Win7 to Win10, would that bring changes to partition 1, necessitating creation of a new Image of that partition, in parallel with an Image of partition 2 with Win10?

Old chinese proverb:If I hear - I forget, If I see - I remember, If I do - I understand

Your WinXP and Win7 booting files are in the WinXP partition. If you upgrade to Win10, the Win10 booting files would be installed into the WinXP partition too. If you wanted to revert to Win7 I'd expect you would have to restore both the WinXP and Win7 images.